In the usual methods for the treatment of finely granulated solids, for example a circulating fluidized bed, residence times are defined by a series of boundary conditions. In the example of the circulating fluidized bed, the residence time is determined by the quantity of the solid contained in the fluidized bed, the pressure drop between the reactor bottom and the upper region, as well as by the feed rate of the solid. In summary, then, the residence time in the circulating fluidized bed is defined as the relationship between the total mass of the moving bed and the mass throughput of the unit.
In the circulating fluidized bed, the residence time for the solid in the fluidized bed can be determined by adjusting the pressure drop, but the window for such an adjustment is limited, because on the one hand the pressure drop in the system could become too high when the charge is correspondingly high, or on the other hand when the drop is too low, a homogeneous bed is no longer formed.
This is also the case for other systems with a defined geometry and a fixed volume in which, in general, the residence time for the solid in the respective reactor system is inversely proportional to the throughput of the unit, which is why large variations in the solid residence time can result in different reaction conditions and thus in substantial fluctuations in the quality of the product.
There is also a problem with the interaction between the capacity of the unit and the residence time, and so, in order to be able to produce consistent residence times, even the mass throughputs can only be operated with slight variations, or the solid residence time has to be capable of being kept substantially constant irrespective of the charge status of the unit.